diff --git a/core/includes/menu.inc b/core/includes/menu.inc
index 2fd7b52..3f1ec20 100644
--- a/core/includes/menu.inc
+++ b/core/includes/menu.inc
@@ -1617,6 +1617,9 @@ function theme_menu_link(array $variables) {
  */
 function theme_menu_local_task($variables) {
   $link = $variables['element']['#link'];
+  $link += array(
+    'localized_options' => array(),
+  );
   $link_text = $link['title'];
 
   if (!empty($variables['element']['#active'])) {
@@ -1863,8 +1866,8 @@ function menu_local_tasks($level = 0) {
   $data = &drupal_static(__FUNCTION__);
   $root_path = &drupal_static(__FUNCTION__ . ':root_path', '');
   $empty = array(
-    'tabs' => array('count' => 0, 'output' => array()),
-    'actions' => array('count' => 0, 'output' => array()),
+    'tabs' => array('output' => array()),
+    'actions' => array('output' => array()),
     'root_path' => &$root_path,
   );
 
@@ -1916,7 +1919,6 @@ function menu_local_tasks($level = 0) {
     // Tab parenting may skip levels, so the number of parts in the path may not
     // equal the depth. Thus we use the $depth counter (offset by 1000 for ksort).
     $depth = 1001;
-    $actions['count'] = 0;
     $actions['output'] = array();
     while (isset($children[$path])) {
       $tabs_current = array();
@@ -1981,9 +1983,7 @@ function menu_local_tasks($level = 0) {
         }
       }
       $path = $next_path;
-      $tabs[$depth]['count'] = $tab_count;
       $tabs[$depth]['output'] = $tabs_current;
-      $actions['count'] += $action_count;
       $actions['output'] = array_merge($actions['output'], $actions_current);
       $depth++;
     }
@@ -2046,7 +2046,6 @@ function menu_local_tasks($level = 0) {
       }
       $path = $next_path;
       $parent = $next_parent;
-      $tabs[$depth]['count'] = $count;
       $tabs[$depth]['output'] = $tabs_current;
       $depth--;
     }
@@ -2183,7 +2182,7 @@ function menu_contextual_links($module, $parent_path, $args) {
 function menu_primary_local_tasks() {
   $links = menu_local_tasks(0);
   // Do not display single tabs.
-  return ($links['tabs']['count'] > 1 ? $links['tabs']['output'] : '');
+  return count($links['tabs']['output']) > 1 ? $links['tabs']['output'] : '';
 }
 
 /**
@@ -2192,7 +2191,7 @@ function menu_primary_local_tasks() {
 function menu_secondary_local_tasks() {
   $links = menu_local_tasks(1);
   // Do not display single tabs.
-  return ($links['tabs']['count'] > 1 ? $links['tabs']['output'] : '');
+  return count($links['tabs']['output']) > 1 ? $links['tabs']['output'] : '';
 }
 
 /**
diff --git a/core/includes/module.inc b/core/includes/module.inc
index ab45576..fc66bb8 100644
--- a/core/includes/module.inc
+++ b/core/includes/module.inc
@@ -31,7 +31,7 @@ function module_load_all($bootstrap = FALSE) {
   $has_run = &$drupal_static_fast['has_run'];
 
   if (isset($bootstrap)) {
-    $type = $bootstrap ? 'bootstrap' : 'module_enabled';
+    $type = $bootstrap ? 'bootstrap' : 'module';
     foreach (module_list($type) as $module) {
       drupal_load('module', $module);
     }
@@ -54,7 +54,7 @@ function module_load_all($bootstrap = FALSE) {
  *
  * @param string $type
  *   The type of list to return:
- *   - module_enabled: All enabled modules.
+ *   - module: All enabled modules.
  *   - bootstrap: All enabled modules required for bootstrap.
  * @param array $fixed_list
  *   (optional) An array of module names to override the list of modules. This
@@ -66,7 +66,7 @@ function module_load_all($bootstrap = FALSE) {
  *   An associative array whose keys and values are the names of the modules in
  *   the list.
  */
-function module_list($type = 'module_enabled', array $fixed_list = NULL) {
+function module_list($type = 'module', array $fixed_list = NULL) {
   // This static is only used for $fixed_list.
   $module_list = &drupal_static(__FUNCTION__);
 
@@ -102,13 +102,13 @@ function module_list_reset() {
  *
  * @param $type
  *   The type of list to return:
- *   - module_enabled: All enabled modules.
+ *   - module: All enabled modules.
  *   - bootstrap: All enabled modules required for bootstrap.
- *   - theme: All themes.
+ *   - theme: All enabled themes.
  *
  * @return
  *   An associative array of modules or themes, keyed by name. For $type
- *   'bootstrap' and 'module_enabled', the array values equal the keys.
+ *   'bootstrap' and 'module', the array values equal the keys.
  *   For $type 'theme', the array values are objects representing the
  *   respective database row, with the 'info' property already unserialized.
  *
@@ -144,13 +144,13 @@ function system_list($type) {
     $lists['bootstrap'] = array_keys($bootstrap_list);
   }
   // Otherwise build the list for enabled modules and themes.
-  elseif (!isset($lists['module_enabled'])) {
+  elseif (!isset($lists['module'])) {
     if ($cached = cache('bootstrap')->get('system_list')) {
       $lists = $cached->data;
     }
-    else {
+    if (!isset($lists[$type])) {
       $lists = array(
-        'module_enabled' => array(),
+        'module' => array(),
         'theme' => array(),
         'filepaths' => array(),
       );
@@ -159,11 +159,11 @@ function system_list($type) {
       // Drupal installations, which might have modules installed in different
       // locations in the file system. The ordering here must also be
       // consistent with the one used in module_implements().
-      $result = db_query("SELECT * FROM {system} WHERE type = 'theme' OR (type = 'module' AND status = 1) ORDER BY weight ASC, name ASC");
+      $result = db_query("SELECT * FROM {system} WHERE (type = 'theme' OR type = 'module') AND status = 1 ORDER BY weight ASC, name ASC");
       foreach ($result as $record) {
         // Build a list of all enabled modules.
         if ($record->type == 'module') {
-          $lists['module_enabled'][$record->name] = $record->name;
+          $lists['module'][$record->name] = $record->name;
         }
         // Build a list of themes.
         if ($record->type == 'theme') {
@@ -171,9 +171,7 @@ function system_list($type) {
           $lists['theme'][$record->name] = $record;
         }
         // Build a list of filenames so drupal_get_filename can use it.
-        if ($record->status) {
-          $lists['filepaths'][] = array('type' => $record->type, 'name' => $record->name, 'filepath' => $record->filename);
-        }
+        $lists['filepaths'][] = array('type' => $record->type, 'name' => $record->name, 'filepath' => $record->filename);
       }
       foreach ($lists['theme'] as $key => $theme) {
         if (!empty($theme->info['base theme'])) {
diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index f807979..7e875bf 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -37,6 +37,34 @@
  */
 
 /**
+ * Loads a theme.
+ *
+ * @param string $name
+ *   The name of a theme to load.
+ *
+ * @return
+ *   The loaded theme object for $name, or FALSE if the passed $name does not
+ *   exist or is not enabled.
+ */
+function theme_load($name) {
+  $themes = list_themes();
+  if (isset($themes[$name]) && drupal_theme_access($themes[$name])) {
+    return $themes[$name];
+  }
+  return FALSE;
+}
+
+/**
+ * Menu title callback; Returns the sanitized human-readable name of a theme.
+ *
+ * @param $theme
+ *   The theme object to return the title for.
+ */
+function _theme_menu_title($theme) {
+  return check_plain($theme->info['name']);
+}
+
+/**
  * Determines if a theme is available to use.
  *
  * @param $theme
@@ -47,11 +75,18 @@
  *   FALSE otherwise.
  */
 function drupal_theme_access($theme) {
+  // Early-return for potentially bogus '0', FALSE, or NULL values.
+  if (empty($theme)) {
+    return FALSE;
+  }
   if (is_object($theme)) {
     return !empty($theme->status);
   }
   else {
     $themes = list_themes();
+    if (!isset($themes[$theme])) {
+      return FALSE;
+    }
     return !empty($themes[$theme]->status);
   }
 }
@@ -652,14 +687,14 @@ function _theme_build_registry($theme, $base_theme, $theme_engine) {
  *     declare this theme as their base theme. 
 */
 function list_themes($refresh = FALSE) {
-  $list = &drupal_static(__FUNCTION__, array());
+  $list = &drupal_static(__FUNCTION__);
 
   if ($refresh) {
-    $list = array();
+    $list = NULL;
     system_list_reset();
   }
 
-  if (empty($list)) {
+  if (!isset($list)) {
     $list = array();
     $themes = array();
     // Extract from the database only when it is available.
diff --git a/core/lib/Drupal/Core/DrupalKernel.php b/core/lib/Drupal/Core/DrupalKernel.php
index 869d4ee..b0a7947 100644
--- a/core/lib/Drupal/Core/DrupalKernel.php
+++ b/core/lib/Drupal/Core/DrupalKernel.php
@@ -45,7 +45,7 @@ public function registerBundles() {
 
     // @todo Remove the necessity of calling system_list() to find out which
     // bundles exist. See http://drupal.org/node/1331486
-    $modules = array_keys(system_list('module_enabled'));
+    $modules = array_keys(system_list('module'));
     foreach ($modules as $module) {
       $camelized = ContainerBuilder::camelize($module);
       $class = "Drupal\\{$module}\\{$camelized}Bundle";
diff --git a/core/modules/block/block.admin.inc b/core/modules/block/block.admin.inc
index aa018da..bbbffbc 100644
--- a/core/modules/block/block.admin.inc
+++ b/core/modules/block/block.admin.inc
@@ -33,6 +33,9 @@ function block_admin_display($theme = NULL) {
     // If theme is not specifically set, rehash for the current theme.
     $theme = $theme_key;
   }
+  elseif (is_object($theme)) {
+    $theme = $theme->name;
+  }
 
   // Fetch and sort blocks.
   $blocks = block_admin_display_prepare_blocks($theme);
diff --git a/core/modules/block/block.module b/core/modules/block/block.module
index 4a3d6b9..a563ca6 100644
--- a/core/modules/block/block.module
+++ b/core/modules/block/block.module
@@ -62,11 +62,21 @@ function block_help($path, $arg) {
     case 'admin/structure/block/add':
       return '<p>' . t('Use this page to create a new custom block.') . '</p>';
   }
-  if ($arg[0] == 'admin' && $arg[1] == 'structure' && $arg['2'] == 'block' && (empty($arg[3]) || $arg[3] == 'list')) {
-    $demo_theme = !empty($arg[4]) ? $arg[4] : variable_get('theme_default', 'stark');
-    $themes = list_themes();
+
+  if ($arg[0] === 'admin' && $arg[1] === 'structure' && $arg['2'] === 'block' && (empty($arg[3]) || $arg[3] === 'list')) {
     $output = '<p>' . t('This page provides a drag-and-drop interface for assigning a block to a region, and for controlling the order of blocks within regions. Since not all themes implement the same regions, or display regions in the same way, blocks are positioned on a per-theme basis. Remember that your changes will not be saved until you click the <em>Save blocks</em> button at the bottom of the page. Click the <em>configure</em> link next to each block to configure its specific title and visibility settings.') . '</p>';
-    $output .= '<p>' . l(t('Demonstrate block regions (@theme)', array('@theme' => $themes[$demo_theme]->info['name'])), 'admin/structure/block/demo/' . $demo_theme) . '</p>';
+    if (!empty($arg[4])) {
+      if (drupal_theme_access($arg[4])) {
+        $demo_theme = $arg[4];
+      }
+    }
+    else {
+      $demo_theme = variable_get('theme_default', 'stark');
+    }
+    if (isset($demo_theme)) {
+      $themes = list_themes();
+      $output .= '<p>' . l(t('Demonstrate block regions (@theme)', array('@theme' => $themes[$demo_theme]->info['name'])), 'admin/structure/block/demo/' . $demo_theme) . '</p>';
+    }
     return $output;
   }
 }
@@ -141,39 +151,66 @@ function block_menu() {
     'type' => MENU_LOCAL_ACTION,
     'file' => 'block.admin.inc',
   );
-  foreach (list_themes() as $key => $theme) {
-    $items['admin/structure/block/list/' . $key] = array(
-      'title' => check_plain($theme->info['name']),
-      'page arguments' => array($key),
-      'type' => $key == $default_theme ? MENU_DEFAULT_LOCAL_TASK : MENU_LOCAL_TASK,
-      'weight' => $key == $default_theme ? -10 : 0,
-      'access callback' => '_block_themes_access',
-      'access arguments' => array($key),
-      'file' => 'block.admin.inc',
-    );
-    if ($key != $default_theme) {
-      $items['admin/structure/block/list/' . $key . '/add'] = array(
-        'title' => 'Add block',
-        'page callback' => 'drupal_get_form',
-        'page arguments' => array('block_add_block_form'),
-        'access arguments' => array('administer blocks'),
-        'type' => MENU_LOCAL_ACTION,
-        'file' => 'block.admin.inc',
+
+  $items['admin/structure/block/list/%theme'] = array(
+    'title' => 'Theme blocks',
+    'title callback' => '_theme_menu_title',
+    'title arguments' => array(4),
+    'page callback' => 'block_admin_display',
+    'page arguments' => array(4),
+    'type' => MENU_LOCAL_TASK,
+    'access callback' => '_block_themes_access',
+    'access arguments' => array(4),
+    'file' => 'block.admin.inc',
+  );
+  $items['admin/structure/block/list/%theme/add'] = array(
+    'title' => 'Add block',
+    'page callback' => 'drupal_get_form',
+    'page arguments' => array('block_add_block_form'),
+    'access arguments' => array('administer blocks'),
+    'type' => MENU_LOCAL_ACTION,
+    'file' => 'block.admin.inc',
+  );
+  $items['admin/structure/block/demo/%theme'] = array(
+    'title' => 'Theme block region demo',
+    'title callback' => '_theme_menu_title',
+    'title arguments' => array(4),
+    'page callback' => 'block_admin_demo',
+    'page arguments' => array(4),
+    'type' => MENU_CALLBACK,
+    'access callback' => '_block_themes_access',
+    'access arguments' => array(4),
+    'theme callback' => '_block_custom_theme',
+    'theme arguments' => array(4),
+    'file' => 'block.admin.inc',
+  );
+  return $items;
+}
+
+/**
+ * Implements hook_menu_local_tasks_alter().
+ */
+function block_menu_local_tasks_alter(&$data, $router_item, $root_path) {
+  if ($root_path == 'admin/structure/block' || strpos($root_path, 'admin/structure/block/list') === 0) {
+    foreach ($data['tabs'][0]['output'] as &$task) {
+      $task['#active'] = FALSE;
+    }
+    $data['tabs'][0]['output'] = array();
+    $default_theme = variable_get('default_theme', 'stark');
+    $current_theme = isset($router_item['original_map'][4]) ? $router_item['original_map'][4] : $default_theme;
+    $themes = list_themes();
+    foreach ($themes as $theme) {
+      $data['tabs'][0]['output'][] = array(
+        '#theme' => 'menu_local_task',
+        '#link' => array(
+          'title' => $theme->info['name'],
+          'href' => 'admin/structure/block/list/' . $theme->name,
+        ),
+        '#active' => $theme->name === $current_theme,
+        '#weight' => $theme->name === $default_theme ? -10 : 0,
       );
     }
-    $items['admin/structure/block/demo/' . $key] = array(
-      'title' => check_plain($theme->info['name']),
-      'page callback' => 'block_admin_demo',
-      'page arguments' => array($key),
-      'type' => MENU_CALLBACK,
-      'access callback' => '_block_themes_access',
-      'access arguments' => array($key),
-      'theme callback' => '_block_custom_theme',
-      'theme arguments' => array($key),
-      'file' => 'block.admin.inc',
-    );
   }
-  return $items;
 }
 
 /**
@@ -208,7 +245,9 @@ function _block_themes_access($theme) {
 function _block_custom_theme($theme = NULL) {
   // We return exactly what was passed in, to guarantee that the page will
   // always be displayed using the theme whose blocks are being configured.
-  return $theme;
+  if (isset($theme->name)) {
+    return $theme->name;
+  }
 }
 
 /**
@@ -412,6 +451,10 @@ function _block_rehash($theme = NULL) {
   }
   $regions = system_region_list($theme);
 
+  if (is_object($theme)) {
+    $theme = $theme->name;
+  }
+
   // These are the blocks the function will return.
   $blocks = array();
   // These are the blocks defined by code and modified by the database.
diff --git a/core/modules/block/lib/Drupal/block/Tests/BlockAdminThemeTest.php b/core/modules/block/lib/Drupal/block/Tests/BlockAdminThemeTest.php
index c3a220f..e80a053 100644
--- a/core/modules/block/lib/Drupal/block/Tests/BlockAdminThemeTest.php
+++ b/core/modules/block/lib/Drupal/block/Tests/BlockAdminThemeTest.php
@@ -39,7 +39,7 @@ function testAdminTheme() {
 
     // Ensure that access to block admin page is denied when theme is disabled.
     $this->drupalGet('admin/structure/block/list/bartik');
-    $this->assertResponse(403);
+    $this->assertResponse(404);
 
     // Enable admin theme and confirm that tab is accessible.
     theme_enable(array('bartik'));
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index 4cf5780..a9a2b2c 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -2604,6 +2604,7 @@ function system_get_info($type, $name = NULL) {
     }
   }
   else {
+    // @todo Migrate theme info to ModuleInfo implementation.
     $list = system_list($type);
     foreach ($list as $shortname => $item) {
       if (!empty($item->status)) {
@@ -2629,6 +2630,8 @@ function system_get_info($type, $name = NULL) {
  *   returned.
  *
  * @see Drupal\Core\Utility\ModuleInfo
+ *
+ * @todo Merge this helper entirely into system_get_info()?
  */
 function system_get_module_info($property) {
   static $info;
@@ -2963,25 +2966,26 @@ function _system_default_theme_features() {
 /**
  * Get a list of available regions from a specified theme.
  *
- * @param $theme_key
- *   The name of a theme.
+ * @param $theme
+ *   A theme object, or the name of a theme.
  * @param $show
  *   Possible values: REGIONS_ALL or REGIONS_VISIBLE. Visible excludes hidden
  *   regions.
  * @return
  *   An array of regions in the form $region['name'] = 'description'.
  */
-function system_region_list($theme_key, $show = REGIONS_ALL) {
-  $themes = list_themes();
-  if (!isset($themes[$theme_key])) {
-    return array();
+function system_region_list($theme, $show = REGIONS_ALL) {
+  if (!is_object($theme)) {
+    $themes = list_themes();
+    if (!isset($themes[$theme])) {
+      return array();
+    }
+    $theme = $themes[$theme];
   }
-
   $list = array();
-  $info = $themes[$theme_key]->info;
   // If requested, suppress hidden regions. See block_admin_display_form().
-  foreach ($info['regions'] as $name => $label) {
-    if ($show == REGIONS_ALL || !isset($info['regions_hidden']) || !in_array($name, $info['regions_hidden'])) {
+  foreach ($theme->info['regions'] as $name => $label) {
+    if ($show == REGIONS_ALL || !isset($theme->info['regions_hidden']) || !in_array($name, $theme->info['regions_hidden'])) {
       $list[$name] = t($label);
     }
   }